There is an urgent need to develop alternative therapies to replace or supplement current antibiotics for treating a whole spectrum of bacterial diseases in view of an alarming increase of antibiotic resistance that poses a very real threat to modern medicine. The emergence of resistant microbial strains, the nature of hospital environments, and the number of routine operations make the spread of infection more hazardous. Additionally, the ease and frequency of international travel assists in the spread of resistant bacteria throughout the world.
Many pathogenic microorganisms reside within biofilms which cause additional problems when designing new anti-microbial agents and therapies. A biofilm is an accumulation of microorganisms embedded in hydrated matrices of cells and containing polysaccharides, extracellular DNA, and proteases. Biofilms may form on solid biological or non-biological surfaces and account for a majority of microbial infections in the body. Bacteria growing in a biofilm rather than in free-floating forms tend to be particularly resistant to anti-microbial agents and it is often particularly difficult for a host immune system to render an appropriate response to bacterial biofilms.
Examples of biofilm-associated microbial infections include infections of oral soft tissues, teeth and dental implants; middle ear; gastrointestinal tract; urogenital tract; airway/lung tissue; eye; urinary tract prostheses; peritoneal membrane and peritoneal dialysis catheters, indwelling catheters for hemodialysis and for chronic administration of chemotherapeutic agents; cardiac implants such as pacemakers, prosthetic heart valves, ventricular assist devices, and synthetic vascular grafts and stents; prostheses, internal fixation devices, and percutaneous sutures; and tracheal and ventilator tubing. Both indwelling and subcutaneous biomedical implants or devices are potential sites for microbial infections and represent important targets for the control of infection, inflammation, and the immune response. Biomedical systems such as blood oxygenators, tracheal lavage, dental water units, and dialyzers are also susceptible to bacterial contamination and biofilm formation.
Pseudomonas aeruginosa (P. aeruginosa) is an opportunistic pathogen known to cause infections in immunocompromised individuals and is the leading cause of mortality among cystic fibrosis (CF) patients (10). The organism possesses a number of virulence factors that contribute to its ability to invade and colonize its host (19, 20) and typically resides in biofilms (5, 9, 13, 14, 23). P. aeruginosa has been shown to form biofilms on abiotic surfaces (e.g. catheters and stents) as well as biotic surfaces (e.g. urinary tract and lung tissue) (2, 7, 18). Biofilms are of significant medical importance because they confer the ability to organisms such as P. aeruginosa to evade the host immune system and render cells more resistant to antimicrobial agents (3, 15). These common characteristics lead to persistent and chronic infections (2).
Photodynamic therapy (PDT) has been useful in the treatment of certain cancers and other diseases such as macular degeneration. In recent years, there has been increased interest in using PDT as a means to treat bacterial infections (22). PDT requires three components: light, oxygen, and a photosensitizer. Light activated cationic porphyrins transfer energy to molecular oxygen resulting in the production of singlet oxygen (1O2). This mechanism is known as the Type II reaction. 1O2 reacts with different components (e.g. phospholipids, peptides, and sterols) of the cell wall and cell membranes and also mediates DNA damage and cell death (21). The cationic porphyrin 5,10,15,20-tetrakis(1-methyl-pyridino)-21H,23H-porphine, tetra-p-tosylate salt (TMP), specifically causes DNA damage by intercalating between DNA base pairs, causing photoinduced strand breakage when irradiated (12, 16).
Previous studies have demonstrated the ability of cationic porphyrins to successfully photoinactivate Gram-positive and Gram-negative bacteria, as well as fungi (11). TMP at a concentration of 2.5 mg ml−1 has been shown to reduce P. aeruginosa PAO1 planktonic cell populations by >102 cfu ml−1 and higher concentrations (5.0 mg ml−1) of TMP were necessary to achieve the same level of killing in bacteria enmeshed within biofilms (8). Additionally, TMP was shown to reduce S. aureus survival and, when combined with vancomycin, to disrupt established biofilms (6).